xfs: xfs_mru_cache_insert() should use GFP_NOFS

xfs_mru_cache_insert() can be called from within transaction context
during block allocation like so:

write(2)
  ....
    xfs_get_blocks
      xfs_iomap_write_direct
        start transaction
        xfs_bmapi_write
          xfs_bmapi_allocate
            xfs_bmap_btalloc
              xfs_bmap_btalloc_filestreams
                xfs_filestream_new_ag
                  xfs_filestream_pick_ag
                    xfs_mru_cache_insert
                      radix_tree_preload(GFP_KERNEL)

In this case, GFP_KERNEL is incorrect and can potentially lead to
deadlocks in memory reclaim. It should use GFP_NOFS allocations to
avoid lock recursion problems.

[dchinner: rewrote commit message]
Signed-off-by: Byoungyoung Lee <blee@gatech.edu>
Signed-off-by: Sanidhya Kashyap <sanidhya.gatech@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_mru_cache.c

@@ -437,7 +437,7 @@ xfs_mru_cache_insert(
 	if (!mru || !mru->lists)
 		return -EINVAL;
 
-	if (radix_tree_preload(GFP_KERNEL))
+	if (radix_tree_preload(GFP_NOFS))
 		return -ENOMEM;
 
 	INIT_LIST_HEAD(&elem->list_node);